The usefulness of the phenomenon of magnetostrictivity in linear distance or position measuring devices is well documented in the prior art. One improvement over the basic technology of particular interest is the increase in measurement resolution as set forth in the U.S. Pat. No. 5,017,867 to Dumais et al entitled "Magnetostrictive Linear Position Detector with Reflection Termination", which is incorporated herein by reference. According to that patent a magnetostrictive wire is stretched between a head and a reflective termination and a displaceable magnet is movably disposed along the wire in accordance with the position to be detected. The wire is excited by an electrical pulse which interacts with the field of the magnet to induce a torsional motion in the wire which propagates as sonic pulses in one direction directly to the head and in the opposite direction to the termination where is reflected to the head. A detector at the head senses the arrival times of the motions which are spaced in time according to the position of the magnet and the propagation velocity along the wire. The difference of the arrival times and the known length of the wire are used to calculate the position of the magnet. Alternatively, the wire is excited by a sonic pulse at the head and an electrical pulse is induced by the magnet when the mechancal pulse reaches the magnet directly and again upon reflection. The timing of the electrical pulses is used to calculate the magnet position in the same way as the electrical excitation type of detector.
The utilization of the entire wire length by the Dumais et al device leads not only to high resolution but also to a sensitivity to thermal changes in wire length. Thermal effects on magnetostrictive detectors have been considered by others as indicated by the United States patents Hunter et al U.S. Pat. No. 5,076,100, Redding U.S. Pat. No. 4,305,283, McCrea et al U.S. Pat. No. 4,158,964, Ueda et al U.S. Pat. No. 4,238,844, Krist U.S. Pat. No. 4,071,818 and Edwards U.S. Pat. No. 4,028,619. Thus it is recognized that the propagation rate of a mechanical pulse in the wire varies with temperature and also that thermal expansion of the wire or the tube supporting the wire can affect measurement accuracy. Various schemes for obviating some of the thermal effects are proposed, including the use of the total propagation times of a mechanical pulse traveling directly between the position magnet and the head and a pulse reflected from an end termination to compensate for changes in propagation velocity. Compensation for changes in wire length is discussed, for example by Krist, but no viable compensation method is advanced. Ueda et al teach measuring the sum of the propagation times along the wire but expressly assume the wire length is constant, using the sum information only for compensation of propagation rate. McCrea et al utilize a bottom reference magnet in a tank to obtain information on wire length. The reference magnet is presumed to have a fixed position but actually the magnet moves relative to the bottom as wire length changes and there is no teaching of compensating for the wire length change. Redding teaches compensating for propagation velocity changes by using two reference magnets at a fixed spacing and a movable magnet between the reference magnets to obtain time ratios independent of the propagation velocity. Hunter et al uses thermistors to provide temperature measurements which are used to adjust distance measurements. Edwards discloses a magnetostrictive rod anchored at one end to provide a reflective termination and has a sonic pulse detector at the other end. The arrangement precludes any effect of thermal expansion on the position measurement, but thermal effects on propagation velocity are detected for compensation purposes by comparing the count representing the sum of direct and reflected pulse propagation times to a constant value.
While effective compensation for thermal changes of propagation rate has been provided in the prior art there has been no effective compensation for thermal expansion of the wire. For the type of high resolution detector employed in this invention, the thermal expansion gives rise to larger errors than the change in propagation rate. For Nispan C magnetostrictive wire, the thermal expansion is 5 ppm/.degree. C. while the propagation velocity change is only 3 ppm/.degree. C. While both effects are significant and must be taken into account where precision measurements are required, the thermal expansion is especially important. Ideally, all thermal effects should be compensated for.
To the extent that the prior art compensates for thermal effects, it does so on the assumption that the magnetostrictive wire is linear; that is a given displacement of the magnet yields a proportionate change in the propagation interval. In reality, the magnetostrictive wires are not perfectly uniform and localized variations in the wire produce anomalies in the measured time intervals. Thus to achieve precise measurements the wire variations should be abrogated along with compensation for thermal effects.